The present invention relates to a burner assembly which can be selectively operated to burn either oil or gaseous fuels therein and which is highly suitable for numerous applications, as for example, in industrial dryers, boilers etc. Of primary importance in all oil or gaseous fuel burners is the mixing of the fuel with the incoming air just prior to combustion as the combustion process is critically dependent on a complete mixing of the fuel and air to achieve a maximum heat release and a clean burn. It is conventional, particularly with the burning of liquid fuels, to achieve this mixing with a highly turbulent action created by high speed rotation or circumferential movement of the combustion air and atomized fuel. This rotation or circumferential movement of air is accomplished by a variety of means such as tangential injection of the fuel and air into a circular burner, by means of stationary veins which impart rotational motion thereto, or the actual use of rotating fans. Unfortunately, uniform mixing does not always result from this rotational or circumferential movement and often stratification of the fuel and air occurs which results in less efficient combustion and often times causes a severe scorching of the burner walls. In addition, the combustion flow resulting from such mixing is often times unmanageable or difficult to control making uniform blending with secondary air for process heating relatively inefficient.
Other features which are desirable in such burner assemblies but which are often times compromised in those assemblies heretofore available include the promotion of a maximum heat generation in the shortest possible time and space thereby to impart the highest concentration of heat to the least amount of combustion air and to reduce the amount of heat loss through the metal walls of the burner by minimizing the area which is a function thereof. It is also desirable to provide a long flow path for the combustion gases to create the maximum "residence time" within the combustion chamber or burner so that the combustion process is complete before the products of combustion are discharged from the burner. This is particularly important in the case of liquid fuels which are atomized for burning because premature discharge into the atmosphere or secondary air often times results in recondensation of unburned fuel causing formation of carbon, undesirable smoke and odors and reduced heat release. It has also been found that the combustion process is further improved by preheating both the combustion air and fuel.
In addition to incomplete mixing of the fuel and air, another shortcoming of many of the burner assemblies currently available is their lack of flexibility. Such devices are generally constructed such that they can operate with only a single type of fuel, i.e., gas or fuel oil. In addition, any of such burner assemblies have a very small turn down ratio, i.e., the ratio of the maximum output to the minimum output with which the burner can effectively operate. By providing a burner assembly which could be selectively operated to burn either gas or fuel oil with the flip of a single switch and which had a large turn down ratio, the flexibility of the burner would be greatly increased not only for multiple applications but in a single installation as well for not only could different fuels be used depending upon availability, but the heat output could be regulated within a wide range to meet varying requirements. It would also be highly desirable to provide a fuel burner which could provide complete combustion in a compact combustion zone with low fuel and combustion air pressures. Many gas utility supplies have very limited pressure available and low combustion air pressure requires less horsepower and, therefore, less fan noise is produced.
Other desirable features of a burner assembly include a reduction in the noise generated by the combustion process itself, which reduction is certainly not found in the conventional burners employing high speed rotational or circumferential mixing. It is also an important requirement of the burner design to provide a strong stable pilot flame for reliable ignition of the main combustion gases. In addition, the pilot nozzle should be positioned so that it is not exposed to destructively high temperatures from the main combustion process. The length of the main combustion flame should be kept to a minimum which requires that most of the combustion be accomplished within the combustion chamber designed for that purpose.